a type of phosphorus fertilizer. Finely ground phosphate rock is gray or brown; insoluble in water, it does not cake, and it is readily dispersed. This fertilizer contains 19–30 percent P2O5 in the form of Ca3(PO4)2, which is all but inaccessible to plants, with impurities of CaCO3 and CaF2. The types of rock that are ground to produce the fertilizer include nodular, granular, and certain karstic phosphorites. Ground phosphate rock is used as a base fertilizer on acid podzolic, gray wooded, and bog soils and on leached chernozems and red soils. It is applied in the autumn before plowing. In an acid medium, the phosphorus in ground phosphate rock is gradually converted into Ca(H2PO4)2·H2O, a form that can be used by plants. Ground phosphate rock is most effective on loamy and clay soils for lupine, mustard, buckwheat, peas, and other crops whose roots acidify the soil. The fertilizer is not used on soils that are well limed and heavily manured. The rate of application varies from 40 to 90 kg of P2O5 per hectare. Ground phosphate rock is also used in preparing composts.
Production of ground phosphate rock in the USSR has grown from 90,000 tons of P2O5 in 1940 and 1950 to 265,000 tons in 1960, 973,000 tons in 1970, and 1,059,000 tons in 1975. Outside the Soviet Union, ground phosphate rock is used primarily in the United States and Canada.
